Recycling of electrochemical cells and batteries is both economically and environmentally important. At the present time greater than 98% of lead acid batteries are recycled. Recycling of rechargeable and non-rechargeable consumer cells, e.g., button, D, C, AA and AAA size batteries that take advantage of Li-ion, Zn-carbon, and Zn alkaline chemistries is also desirable.
Currently recycling is dominated by pyrometallurgical refining. Pyrometallurgical processes are not environmentally optimal, as they cause carbon dioxide emissions and generate waste materials, such as slag and dross. These methods of reprocessing spent materials are also costly due to the high energy intensity of pyrometallurgy.
In current recycling practice the spent batteries and cells are first sent to a breaking or shredding operation where they are subjected to mechanical comminution. Polymeric materials used in the cell casing are removed from the crushed batteries by a sink/float operation in which the low density plastics are floated away from the other materials due to density differences. In the case of lead acid batteries, the paste is then processed in a pyrometallurgical operation in which the materials are heated to >1000° C. in a chemical reducing atmosphere. In this operation the lead based compounds (i.e., PbSO4, PbO2, PbO) are chemically reduced to metallic lead which is removed for further metallurgical refining. These are very energy intensive processes, particularly in light of the fact that much of the lead produced in this operation is re-converted to lead oxides for use in the manufacturing of new lead acid batteries.
Carbon is present in many batteries as an electrochemically active material, such as an anode in a Li-ion cell, or as a modifier to improve electrical conductivity in the electrochemically active material, or to add a capacitive element to the battery to improve charge/discharge properties. When pyrometallurgical recycling techniques are used, the carbon can lead to excess carbon dioxide emissions and difficulty in maintaining the proper CO2/CO ratio for effective smelting. Conventional pyro- and hydro-metallurgical recycling processes for these cells also often render the carbon inactive, making it unsuitable for reuse in the construction of new batteries.
During the recycling process the presence of carbon can also limit the effectiveness of leaching, electrostatic and density driven separation processes. This has been found in recycling of both lead acid and non lead acid batteries, e.g., Li-ion, nickel-metal hydride and zinc based batteries. Specifically, in the case of leaching, where chemically active solutions are used to recover and separate metallic species, carbon can form colloidal suspensions which remove the leachant from the process. Carbon can be contaminated by the reagents used in hydrometallurgical operation, making disposal environmentally difficult. Therefore, removal of carbon prior to the recycling of batteries is beneficial.
While froth flotation has been used in other fields, it has not had application in the field of battery and electrochemical cell recycling. It has now been discovered that by using froth flotation technology to separate certain compounds during recycling of batteries, the thermochemical reduction step used in current recycling processes can be avoided. When froth flotation processes are used, the cost of producing recycled material suitable for reuse in the construction of new lead based electrochemical cells relative to pyrometallurgical processes can be reduced. Use of the froth flotation technique in recycling also has a reduced environmental impact relative to prior art pyrometallurgical techniques, as undesirable emissions are reduced or eliminated. When carbon is removed by froth flotation, the disadvantages associated with carbon can be avoided. In particular, carbon separated by froth flotation can be used directly in battery manufacture. Likewise, other battery materials separated by froth flotation are also of a grade suitable for direct use in battery manufacture.